1. Field of the Invention
The present invention relates to a moving picture experts group (MPEG) system, particularly to a technique of searching I picture of MPEG data at a high speed.
2. Description of the Related Art
In the MPEG system for compressing and transmitting image information, the image information are mixed and transmitted as three types of image data: I picture; P picture; and B picture. Here, the I picture indicates an intra-frame coded image, and one frame can be reconstructed singly with the information, without requiring other frame information for deciding the image. The P picture indicates an inter-frame forward direction predictive coded image, and the information of earlier I picture of P picture is necessary for decoding the image. The B picture indicates an inter-frame bi-directional predictive coded image, and the information of earlier and later I picture of P picture is necessary for decoding the image.
FIG. 3 shows the layer structure of MPEG video data. In FIG. 3, a sequence layer indicates a topmost layer, and a group of picture (GOP) constituted by disposing the I picture on the top and also setting P and B pictures, and a sequence header (SH) indicating header information for controlling each GOP are alternately arranged. Moreover, in the subsequent GOP layer, various pictures constituting each GOP are arranged.
In the image playback of the MPEG system, when quick forwarding, or another special playback is performed, the above-described fact that one frame can be regenerated with the I picture alone is utilized, and a method comprising skipping the P and B pictures in a data stream and extracting and regenerating only the I pictures is performed.
However, in the MPEG system, since an information compression ratio differs with the frames, I picture intervals are not constant. Specifically, intervals X1, X2, and X3 to the next I picture shown in FIG. 3 are not constant.
Therefore, in order to extract the I pictures from the MPEG video data, a method comprising searching all the data to extract the I pictures has heretofore been used. The conventional I picture search method will be described hereinafter.
First, a method of decoding/processing the MPEG video data during the quick forwarding playback will be described. FIG. 4 is a block diagram showing the configuration of a decoding processor of the MPEG video data.
An I picture searcher 101 extracts I picture data from the MPEG video data. The extracted I picture data is subjected to variable length decoding in a variable length decoder (VLD) 102, and inversely quantized in an inverse quantizing unit 103, and an inverse discrete cosine transformer (IDCT) 104 performs an inverse discrete cosine transform to convert a frequency area to a space area, so that original image data is regenerated.
The details of the I picture searcher will next be described. FIG. 1 is a block diagram showing the configuration of a conventional I picture search apparatus.
The MPEG video data accumulated in a video memory 1 is successively written to a buffer 4 in accordance with an address supplied from an address register 2. The data read from the buffer 4 at a predetermined timing is compared with a specific bit pattern corresponding to the I picture in an I picture detector 5 to determine whether or not the data is I picture. As a determination result, when the data is the I picture, the data is outputted from a data output unit 6.
In this case, an address controller 3 increments the address by the amount of read data, generates the next data address, and stores the address in the address register 2, so that the I pictures are successively searched.
As described above, even when only the I picture data is necessary, the conventional I picture search method comprises successively reading all the picture data in the MPEG video data and checking whether or not the data is the I picture to extract the I pictures.
However, it requires a remarkably long processing time to check all the picture data constituting the MPEG video data in order to extract the I pictures from the MPEG video data, and a problem occurs that the processing is inefficient.
The present invention has been described in consideration of the above-described situation, and an object thereof is to provide an I picture search apparatus of MPEG data in which a processing of extracting I pictures from MPEG video data can be speeded up.
The MPEG video data stored in a video memory is successively written to a buffer in accordance with an address held in an address register. When I picture is detected from the data read from the buffer, the information is fed to an I picture interval holder. The I picture interval holder holds an address register value when the previous I picture is detected, subtracts the previous address register value from the present address register value when the present I picture is detected, and holds the value (interval between the previous I picture address and the present I picture address). A next I picture address calculator adds the interval obtained in the I picture interval holder to the present address value stored in the address register, subtracts a predetermined value from the calculated value, and predicts the next I picture address.
Therefore, since the I picture can be extracted from the MPEG video data without searching all the data in the MPEG video data, the processing of extracting the I picture from the MPEG video data can be speeded up.